Juan Valentin and Jack Guidone begin their cabaret-style review, "Back to Back," with a rambling dialogue on mellowness - one thing they both have in good measure.

The best parts of their show, which will be at the New Playwrights' Theatre tonight and tomorrow night, are light numbers that allow them to show their considerable dancing agility and the pleasant tone of their voices - particularly duets on "Well, Did You Evah," "Fascinatin" Rhythm" and "You Gotta Have a Gimmick."

One of their gimmicks that did not work was costumes - tight to the point of obscenity and, no doubt, discomfort - that reduced a small female cheering section to wild giggles.

The act still has some growing to do, as the partners showed, for example, in a song from "Man of La Mancha" - a bland rendition far short of the music's dramatic potential. But there is a chance for growth; they have what you must be born with, and what they need can be acquired by hard work.

At this point, still fairly early in their career, Valentin and Guidone are two disparate talents seeking a joint character and identity - something more compelling and distinctive than the (quite accurate) image of two guys with good voices who know some great songs. They have not yet found it, they are proceeding by trial and error and their errors can be a bit of a trial.

"Back to Back" is strung loosely together with a dressing-room setting before the intermission and an opening-night party motif in the second half. The party routine has some clever touches - for example, two female mannequins with whom the song-and-dance men try vainly to make time - but these resources are not really exploited. All that can be said is that they are a slight improvement on stepping forward and saying flatly, "For my next number, I will sing..."

As for their individual assets, Guidone seems to have slightly more natural vocal ability, but Valentin does quite a bit more with his slightly less mellow tone - his sizzling and excellently phrased "Fever" was probably the highlight of the evening. With some work on phrasing and expression, Guidone could become a singer worth a lot more attention.